In the production of seed potatoes in hydroponics, it is most important to produce disease-free potato seedlings, which are not infected with viruses and do not have seed-transmissible diseases, and to mass produce high quality potato seedlings within a short period of time.
Once potato seedlings planted in culture beds for producing seed potatoes become infected with viruses or develop major plant diseases, seed potatoes obtained from the infected seedlings are also infected with viruses or have major plant diseases through permanent seed transmission, thereby losing their value as seed potatoes.
Thus, potato seedlings, which are used as starting materials in the production of disease-free seed potatoes, should be obtained from virus-free seedlings, which are propagated through in vitro tissue culture.
In addition, tissue-cultured seedlings, which are produced under culture conditions of high humidity and low light levels, mostly wither and die when they are removed and immediately planted in the external air because the seedlings are grown in an in vitro-culture vessel, which has a low flow of gas and energy and a small temperature change but has a relatively high humidity and a large diurnal change of CO2 concentration. The CO2 concentration inside the closed culture vessel is about 100 μmole·m-1, which is lower than 350 μmole/m·-2·s-1, during the day, but sharply increases to 3,000-9,000 μmole·m-1 in the dark. A large quantity of ethylene gas is generated in the vessel because the culture medium is supplemented with sugars, salts and growth regulators and the culture vessel is completely closed to exclude external contaminants and thus prevent contamination. Also, since seedlings growing in the vessel have low growth rates due to the high humidity inside the culture vessel and thus have morphology characterized by long internode lengths, thin stems and narrow leaves, they are not suitable for planting in hydroponics using a nutrient solution.
In vitro propagation through tissue culture, that is, solid culture, liquid culture or tank culture, incurs a high cost for seedling production. This is because the mass production of genetically homogenous seedlings generates completely functional individual plants through culture under suitably sterile conditions, and thus is achieved with facilities having expensive equipment and maintained under suitable conditions. Also, since the entire process is done manually and depends on experience, the labour cost accounts for about one half of the production cost. Also, plantlets grow slowly during propagation and are unable to adapt to the external environment during the acclimatization process, resulting in low survival rates.
To be planted in greenhouse or other culture systems with a nutrient solution, plantlets growing in culture bottles need to be stimulated to grow at the maximum rate and to grow into robust seedlings capable of surviving external stresses when exposed to the external environment. Thus, it is important in the production of potato seedlings to improve the survival rates and quality of plantlets by suitably controlling the acclimatization of plantlets using an artificial method.
Potato seedlings may be produced by a method in which in vitro-cultured plantlets are acclimatized to become potato seedlings, which are then planted, or by another method in which minitubers weighing less than 5 g, which are more difficult to provide for direct sowing in field of in vitro-produced artificial seed potatoes or hydroponically grown seed potatoes, are awoken from dormancy, allowed to sprout and grow to about 7-8 cm in a sterile medium (perlite), and immediately planted as seedlings for planting in hydroponics after mother potatoes are removed therefrom.
The acclimatization of in vitro-cultured plantlets to obtain potato seedlings can be achieved using a method of producing seed potatoes, through on cutting and planting in vitro-culture acclimatized plantlets in sterile perlite culture to form new roots, or using a method of producing seed potatoes by acclimatizing in vitro-cultured plantlets in deep flow hydroponics to which a nutrient solution is supplied through a predetermined procedure and planting the acclimatized plantlets after conducting stem cutting.
The production of potato seedlings through culture in perlite is achievable by allowing plantlets to grow in in vitro culture bottles in a greenhouse having the same environment as a culture room, in which the temperature and light intensity are slowly increased, and air humidity is controlled by widening holes in the entrance. The in vitro-cultured plantlets thus acclimatized are removed from the culture bottles, rinsed with pure water, and planted in a small-scale sterile perlite container for raising seedlings through cut planting or stem cutting to develop new roots on the stems, thereby functioning as seed potatoes. However, there is a very significant problem with this system, as follows. When the planted potato seedlings are exposed to high air humidity or frequent water supply during acclimatization, stems at regions of the plant that contact the perlite in order to develop new roots become very soft and rot. Thus, rooted individuals are poorly acclimatized, and thus show low survival. Also, since very high light levels cause stems to wither, growers should suitably control the light intensity during acclimatization according to the states of plantlets based on experience and judgment. Moreover, the perlite used for acclimatization must be disposed of due to contamination with pathogens, spreading of pathogens, excessive residual nutrients, and the like, and thus is a new environmental contaminant.
The soft rot and withering occur due to changes in growth environment and the nature of in vitro-cultured plantlets. Since the in vitro culture system has a relative humidity of 90-100%, in vitro-grown plantlets have poor cuticle wax layers, and have smaller and fewer palisade cells than common plants. Also, due to the high humidity, the stomata on leaves of the in vitro-grown plantlets always remain open, and thus are not functional. Roots and stems have poor vascular connections, and thus in vitro-developed roots possess few or no root hairs. When solid medium-grown seedlings are removed from the in vitro conditions, roots extending into the medium are greatly damaged, and the in vitro-cultured seedlings thus cannot perform fully their original function.
An alternative method of acclimatizing in vitro tissue-cultured plantlets into potato seedlings is based on manufacturing a culture bed using a molded bed of Expanded polystylene and a plastic container box, planting cultured plantlets in an upper board of the culture bed covered with a black vinyl material to prevent internal leakage, and supplying oxygen to a nutrient solution in a manner of supplying nutrients using an air pump filled with the nutrient solution, thereby producing potato seedlings. However, this method also has practical limitations in the mass production of seed potatoes. Due to the nature of in vitro-cultured plantlets and changes in the growth environment, the cut regions of stem cuttings of potato plantlets rot, stems including growing points wither, and so on, thereby making the plantlets unable to grow. Since in vitro-cultured plantlets utilize the limited nutrients contained in the nutrient solution, they lack specific growth nutrients. For a period of about 30 to 40 days required for the production of potato seedlings from in vitro-cultured plantlets, seedlings shocked by nutrient deficiency must be planted in hydroponic facilities after being diagnosed according to the expressed nutritional disorder symptoms and treated through the supplement of deficient nutrients. However, to shorten the acclimatization and cultivation period, the shocked seedlings are immediately planted in hydroponic facilities, causing growers great loss. Also, since seedlings planted through stem cutting directly take up nutrients contained in the culture bed, they are susceptible to contamination by pathogens and to decay. In this case, all of the growing seedlings must be weeded out or discarded, wreaking havoc on potato crops for that year.
Since technical skill and experience from failure lead to success in the production of potato seedlings from in vitro-cultured plantlets as described above, beginning growers have difficulty in practice obtaining high quality potato seedlings. For this reason, in order to stably obtain potato seedlings, seedling sprouts, which are obtained by breaking the dormancy of seed potatoes having a predetermined size (minitubers) and sprouting the minitubers, are used. The seedling sprouts are convenient for use as potato seedlings for planting due to the following advantage. When minitubers having a size of lower than 5 g, which are difficult to seed directly in field of in vitro-produced artificial seed potatoes and hydroponically grown seed potatoes, are dormancy broken, allowed to sprout and grow to about 7-8 cm in stem height in a sterile medium (perlite), and immediately planted as seedlings for planting in hydroponics after mother potatoes are removed therefrom, they can be immediately planted as young seedlings in hydroponics without the complicated acclimatization process required for in vitro-cultured plantlets.
However, there are drawbacks with the use of seedling sprouts. Since minitubers, like general potatoes, have various degrees of dormancy and dormancy periods according to the cultivar, they must be used after conducting the cumbersome process of dormancy breaking. Also, when seedling sprouts are generated using perlite and bed soil, they are susceptible to viral contamination and other major diseases in seed potatoes, leading to lower quality than initial tissue cultured-seedlings. Further, when minitubers to be used to generate seedling sprouts have poor or uncertified quality, they are unable to be used as potato seedlings.
It is also important to produce potato seedlings which are tall and have many nodes on which stolons are formed in order to produce potato seedlings suitable for hydroponics.
The cultivation method of producing seed potatoes by culturing potato seedlings using a nutrient solution is based not on immersing potato seedlings in the nutrient solution or culturing them in perlite or bed soil, but on planting potato seedlings in the air by hanging them on a hollow Expanded polystylene culture bed. Since potato seedlings are inserted into slits formed on the side surface of a sponge in an up to down direction and pushed along with the sponge into planting holes on the upper board of a culture bed. In the case of weak or over-grown potato seedlings, even when growers with much planting experience transplant potato seedlings, even carefully, the planted seedlings are easily broken and bent inside the sponge, eventually withering and dying. Further, since the planted seedlings do not wither due to water contained therein although they are caught in the sponge, growers cannot find such seedlings. When such seedlings wither, wilt or die due to water loss in the external environment as time goes, the growers find seedlings shocked by environmental stresses caused by planting states, and replant seedlings to replace the shocked seedlings with new seedlings. Thus, a lot of labor and many potato seedlings are required and wasted for replanting for a period ranging from the planting date to the day on which a desired stand of seedlings is established. Also, due to the uneven growth rates according to different rooting time for the growing potato seedlings and the newly planted potato seedlings, required nutrient concentrations and solution supply differ, and some seedlings display immature growth when converted to the reproductive growth period, resulting in low stolon generation. The resulting low tuber formation reduces overall yields and prolongs growth periods, thereby imposing a burden on growers, to perform cultivation management, and causing related stress.
In addition, when produced potato seedlings are short, few or no stem nodes are exposed to the root zone below a culture bed due to the thickness of the upper board of the culture bed, which serves to support potato seedlings, and the thickness of the sponge into which seedlings are inserted. Thus, planted seedlings have lower opportunity for rooting and stolon generation, resulting in decreased yields of seed potatoes. This is because the upper board of the compressed Expanded polystylene culture bed through which planting holes are formed is about 5 cm thick, and the sponge into which the seedlings are planted in a state of being inserted thereinto has the same thickness of 5 cm as the compressed Expanded polystylene culture bed, or is slightly thicker.
Thus, it becomes more important to produce seedlings having long stems and many nodes, in which stolons, from which potatoes hang, are generated, the nodes exposed to the root zone below the culture bed except for the thickness of the sponge for planting. Such potato seedlings may be produced by over-growing seedlings in a tender state in a dark room after being acclimatized, or by increasing the growth space. The former solution has the following problems. Seedlings have tender stems due to low light levels and thus bend and fall over due to their weight when grow past a certain level. Since such seedlings are easily broken or bent inside the sponge when planted in facilities, they grow poorly or die, leading to low survival rates and poor seedling establishment. In the case of the latter, since the extended space enables sufficient photosynthesis, uniformly grown seedlings, which have an inverted triangular or diamond-shaped appearance of being short in plant height and having strong stems and short nodes, are obtained, thereby ensuring stable rooting. However, the latter method is also problematic in that it does not guarantee a desired number of stolons through the establishment of a sufficient number of nodes because the hydroponics method employs sponges. In contrast, when seedlings are over-grown and thus appear to have a high enough node number to be able to form the maximal number of stolons, stems are hollow and angular, and thus change to nodes which are unable to form stolons from which potatoes will hang, and generate leaves and stems instead.
Therefore, there is an urgent need for the development of a method of mass producing high quality potato seedlings suitable for the mass production of seed potatoes in hydroponics.